The present invention relates to a method of automatically controlling the growth of a neck portion between a seed crystal and a corn portion of a single crystal, which is applicable to an apparatus for growing a single crystal by the Czochralski method.
In apparatus for growing a single crystal by the CZ method, there is an established method of automatic control which is applicable only to those cylindrical sections of the grown single crystal that are grown after a corn section. The growth of the neck portion of the single crystal, however, which is grown since the initiation of the growth of the crystal until the growth of a corn section, is controlled manually by a skilled operator. This is because there are elaborate control requirements to expel dislocations from the crystal surface in the neck portion. For example, in the neck portion the diameter of the grown crystal should be reduced to a value of about 2-5 mm; the crystal should be pulled up at a relatively high speed of not lower than 2 mm/min.; the absolute value of the diameter deviation should be controlled to be not larger than about 0.5 mm and the reduced portion should be grown to a length which must be more than 10 times as large as that diameter. The process of growing the crystal into a desired configuration of the reduced portion, that has to get free of dislocations which is to be followed by unusual increase in the crystal diameter, is so hard a process that even a skilled operator will be able to succeed in such process at the rate of as low as 10%. If the crystal diameter is too reduced, then the lower end of the crystal can be often disconnected from the melt surface with the result that the growth can no longer be continued, or if not disconnected the reduced diameter portion cannot be strong enough to support a cylindrical portion which will normally be grown. If, on the contrary, the crystal diameter is too thick, then dislocations cannot be expelled out sufficiently with the result that the growing process cannot proceed to the step of growing the corn section.